Darwin reduced the principles on which the expression of the emotions depends to the following three:
- The principle of serviceable associated habits.
- That of antithesis.
- That of actions due to the constitution of the nervous system, independently from the first of the will and to a certain extent of habit.
According to Darwin the expression of pain depends essentially on the first and third of these principles. He assumes, indeed, that in all animals, in the series of innumerable generations, intense pain has produced the most violent and diverse movements in order to escape its cause.
As the muscles of the thorax and of the vocal organs are those most habitually used, they were called more particularly into action, and animals began to make sounds, to howl and to screech. Darwin believed that vocal sounds were useful to animals, particularly to the young and to those living in community, because in case of danger the cries serve to call the parents or to warn the other animals. These opinions of Charles Darwin open up a wide field for discussion, but for the present I intend to speak only of the expression of the face, so as to confine my subject within reasonable limits.
The movements of the facial muscles depend, according to Darwin, on the constitution of the nervous system. We must remark, however, that Darwin took this idea from Herbert Spencer, who, a few years before Darwin published his book, had written a chapter in his 'Principles of Psychology’ entitled 'The Language of the Emotions.’ Darwin recognised the priority of Herbert Spencer, and I think it advisable to quote a passage from his book, 'The Expression of the Emotions,’ so that the reader may become acquainted with one of the most important pages published on the subject of which we are treating:
'As Mr. Herbert Spencer remarks’ (says Darwin on p. 71), 'it may be received as an “unquestionable truth that, at any moment, the existing quantity of liberated nerve-force, which in an inscrutable way produces in us the state we call feeling, must expend itself in some direction—must generate an equivalent manifestation of force somewhere”; so that, when the cerebro-spinal system is highly excited and nerve-force is liberated in excess, it may be expended in intense sensations, active thought, violent movements, or increased activity of the glands. Mr. Spencer further maintains that an “overflow of nerve-force, undirected by any motive, will manifestly take the most habitual routes; and, if these do not suffice, will next overflow into the less habitual ones.” Consequently, the facial and respiratory muscles, which are the most used, will be apt to be first brought into action; then those of the upper extremities, next those of the lower, and, finally, those of the whole body.’
The simplicity of this theory is seductive, but it suffices to subject it to a superficial examination in order to see that it does not quite correspond to the facts. If we find out which are the most habitual routes of nerve-force, and write them down in order one after the other, and then compare them with the movements expressing the passions, we shall see that there is not a perfect correspondence.
It was perhaps for this reason that Herbert Spencer afterwards introduced the idea of the nervous lines of least resistance, in order to explain the greater facility with which certain muscles contract, compared to others. 'The molecular motion’ (says Spencer) 'disengaged in any nerve-centre by any stimulus, tends ever to flow along lines of least resistance throughout the nervous system.’
The solution of the problem was thus removed to the domain of experimental physiology. The question now is, whether in reality the muscles most commonly in use are those which have nerves offering less resistance, or whether the nervous excitement hidden in the centres is sufficiently strong to allow the resistance made by the nerves to its passage towards the muscles to be disregarded.